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ABSTRAK Gempabumi dapat menimbulkan bahaya likuifaksi yang dapat merusakkan bangunan dan sarana infrastruktur khususnya di wilayah perkotaan di Indonesia. Investigasi geoteknik bawah permukaan telah dilakukan di daerah Patalan, Bantul, Yogyakarta, untuk mendapatkan gambaran susunan lapisan tanah dan kekuatannya, menentukan kedalaman dan ketebalan lapisan tanah yang berpotensi likuifaksi dan penurunan lapisan tanah akibat likuifaksi. Analisis potensi likuifaksi dilakukan menggunakan data CPT (cone penetration test) dan N-SPT (standard penetration test), dengan mempertimbangkan nilai percepatan getaran tanah maksimum (p.g.a) sebesar 0,25g, magnitudo gempabumi sebesar 6,2 SR dan muka airtanah setempat. Hasil penyelidikan menunjukkan bahwa lapisan tanah lepas selama gempabumi terdiri dari pasir lanauan dan lanau pasiran pada kedalaman antara 0,2 -12,8 m. Analisis potensi likuifaksi mengindikasikan bahwa ketebalan lapisan tanah yang berpotensi terlikuifaksi bervariasi antara 0,2 m dan 5,2 m, Sedangkan penurunan total lapisan tanah terutama terkonsentrasi di wilayah bagian tengah yang terletak di jalur Patahan Opak dengan besaran antara 0,21 cm hingga 12,98 cm.

The September 30, 2009 Padang earthquake has resulted in damages to infrastructures and buildings and a death toll of 383 in Padang City. Numerous liquefaction and ground deformations caused by the earthquake were particularly evidence in the areas few kilometers from the coast. The paper presents results of the previous field geotechnical investigations of liquefaction potential and the recent liquefaction observations in Padang City. A microzonation map was created using the data from liquefaction potential analyses and liquefaction potential indexes. The predicted liquefaction susceptibility zones showed a good agreement with site observations. The assessment suggests that the liquefaction susceptibility decreases to the northeast away from the coastal line.

Pengetahuan mengenai kerentanan likuifaksi di suatu wilayah kota pesisir yang rawan gempabumi besar sangat diperlukan dalam perencanaan tataruang untuk mengurangi bencana seismik. Makalah ini menyajikan hasil evaluasi potensi likuifaksi di wilayah Kota Banda Aceh berdasarkan metode uji penetrasi konus untuk menghasilkan mikrozonasi kerentanan penurunan lapisan tanah akibat likuifaksi di wilayah ini. Hasil analisis menunjukkan bahwa potensi likuifaksi terdapat pada lapisan tanah pasir dan campuran pasir dan lanau, yang dicirikan oleh nilai tahanan konus dan hambatan setempat masing-masing lebih kecil dari 15 MPa dan 150 kPa pada kedalaman dan ketebalan yang bervariasi. Berdasarkan hasil perhitungan penurunan tanah, wilayah Banda Aceh dapat dibagi menjadi lima zona kerentanan. Zona kerentanan tinggi terutama terdapat di Kecamatan Kuta Alam dan Syah Kuala, sedangkan zona kerentanan rendah terutama terdapat di wilayah Kecamatan Banda Raya. Dengan demikian, investigasi geoteknik detil sangat diperlukan untuk mencegah kerusakan pada bangunan dan infrastruktur akibat likuifaksi di wilayah Kota Banda Aceh.

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Land subsidence is a global threat to coastal areas worldwide, including the North Java coastal area. Of many areas experiencing land subsidence in North Java, the rate of land subsidence in Pekalongan has matched the high subsidence rates usually found in big cities. The rate of land subsidence in Pekalongan far exceeds the sea-level rise, resulting in a looming threat of land loss. The devastating impacts of land subsidence are the manifestation of its subsurface movement. Therefore, it is essential to understand the subsurface to elucidate the mechanism of land subsidence. Previous studies on land subsidence in Pekalongan are mainly related to subsidence rate monitoring and have not elaborated on the subsurface condition. This paper reviews the Pekalongan subsurface geology based on available literature to provide insight into the land subsidence problem. The results revealed that the land subsidence occurs in the recent alluvial plain of Pekalongan, consisting of a 30-70 m thick compressible deposit. Possible mechanisms of land subsidence arise from natural compaction, over-exploitation of confined groundwater, and increased built areas. As the seismicity of the study area is low, tectonic influence on land subsidence is considered negligible. It is expected that the offshore, nearshore, and swamp deposits are still naturally compacting. As the surface water supply is minimal, over-exploitation of groundwater resources from the deltaic and Damar Formation aquifers occurs. In the end, future research direction is proposed to reduce the impacts of the subsidence hazard.

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